A flat panel display (FPD) such as liquid crystal display (LCD) or organic light-emitting diode (OLED) is manufactured by coating a thin film transistor (TFT) or various types of transparent multi-thin films on a transparent substrate. A transparent substrate such as a flat glass substrate or a transparent film is used as a basic plate of a flat display. Accordingly, since a thickness variation of the transparent substrate needs to be measured accurately, thickness of the transparent substrate needs to be monitored with high accuracy. Moreover, since a transparent substrate whose thickness needs to be measured is increasing in size, it is becoming difficult to measure the thickness of the transparent substrate.
Thickness measuring methods of a transparent substrate are classified into contact type and noncontact type. However, contact-type measuring methods are limited in measurement speed. Therefore, optical thickness measuring methods that are noncontact-type measuring methods are widely used for in-line measurement on the process of production.
A thickness measurement interferometer is a representative thickness measuring apparatus. The thickness measurement interferometer may analyze an interference signal generated by two beams respectively reflected from a front surface and a back surface of a transparent substrate to measure glass thickness. Since the thickness measurement interferometer uses interference of laser, the thickness measurement interferometer may provide high accuracy and high resolution. However, due to 2 π ambiguity that a phase of the interference signal has, the thickness measurement interferometer may be applied only to a case where thickness variation is small. The thickness measurement interferometer may provide an accurate thickness value only when the phase of the interference signal is measured at a much smaller interval than a spatial frequency of thickness variation and phase unwrapping is performed. Thus, the thickness measurement interferometer may not set a thickness measurement interval without limitation. In addition, the thickness measurement interferometer cannot measure thickness of a film when the film is coated only on some regions of the transparent substrate.